The present invention relates to a method and apparatus for recycling thermoplastic polymers and in particular to a method and apparatus for densifying an expanded thermoplastic polymer.
Polymers can be classified as thermoplastic or thermosetting polymers. Thermosetting polymers are polymers that have sites on their chains at which cross-links between chains are formed when the polymer is heated for the first time. Such polymers can be heated once to softening and molded into particular shapes. The polymers retain these shapes on cooling. Because of the cross-linking that occurs at the elevated temperature, reheating does not reverse the process; instead of becoming soft, the thermosetting polymers degrade or decompose upon reheating. Thermoplastic polymers lack sites on their chains at which cross-links between chains can be formed. These polymers soften when heated and can be molded into desired shapes the polymers retain on cooling. Moreover, the thermoplastic polymers have a linear macromolecular structure that is able to undergo repeated cycles of heating, softening, reshaping and cooling. Thus, thermoplastic polymers can be recycled, for example, by adding a certain percentage of previously-used material when manufacturing new articles from the same type of thermoplastic polymers. Certain products may be made completely of previously-used and recycled thermoplastic polymers. Examples of thermoplastic polymers include polystyrenes, polyethylenes, polypropylenes, polyvinyls, nylons and cellulosics.
An expanded thermoplastic polymer may be prepared by mixing the polymer and a blowing agent at elevated temperature and pressure, and extruding the mixture through a suitable aperture into a zone of lower pressure, for example, atmospheric pressure. At the lower pressure the blowing agent, which is typically a low-boiling, volatile liquid or a compound that exists as a liquid at the elevated temperature and pressure conditions, vaporizes, forming a multiplicity of gas-filled cells and expanding the thermoplastic polymer. When an expanded thermoplastic polymer is reheated to the softening range of the polymer, the gas-filled cells are collapsed, and the expanded thermoplastic polymer is densified. As used herein, the softening range is the temperature range in which a thermoplastic polymer goes from a rigid to a soft condition.
Expanded thermoplastic polymers, which are also known as plastic foams, are used extensively to make molded and loose fill packaging material, and disposable cups and food containers. An expanded thermoplastic polymer such as expanded polystyrene is inexpensive to produce, is light and durable, and has excellent insulating properties. Unfortunately, although thermoplastic polymers can be recycled, the expanded thermoplastic polymer has such a low density, and consequently occupies such a large volume, that it is not cost-effective to transport the material to a recycling center. As a result, millions of metric tons of expanded thermoplastic polymers end up in landfills rather than recycling centers.
U.S. Pat. No. 5,286,321 to Fuss discloses a method for densifying expanded thermoplastic materials by confining the material in a vertical mesh chute and heating the material in the lower portion of the chute to collapse and densify the expanded thermoplastic material. However, this apparatus requires all of the expanded thermoplastic particles to be about the same size because if a range of particle sizes is introduced into the chute, a heat gradient is created which results in some particles receiving insufficient heat to densify the particles while other particles may be overheated and begin to degrade or decompose. In addition, a person or a device is required to evaluate the densified material and determine when the material is sufficiently densified and ready for removal from the chute.
Thus, there is still a need for a method and apparatus for densifying expanded thermoplastic polymers which can process a range of particle sizes efficiently and economically.
According to the present invention, such a need is satisfied by introducing particles of a thermoplastic polymer into a chamber, suspending the particles with a fluid flow in the chamber, and while suspending the particles, heating and softening the particles to form pieces of densified thermoplastic polymer which are too dense to remain suspended in the fluid flow. Thus, a range of particle sizes can be efficiently densified without adjusting the apparatus because the smaller or lighter particles quickly densify and are no longer suspended in the fluid flow while the larger particles remain suspended until each one has absorbed a sufficient amount of heat to densify the larger particle. Thus, a wide range of particle sizes can be densified effectively and continuously without outside intervention by a human operator. Expanded thermoplastic polymers having a density as low as 2.4 kg/m.sup.3 can be densified.